While angioplasty has gained wide acceptance, abrupt closure and restenosis have been identified as possible subsequent occurrences. Abrupt closure refers to the acute occlusion of a vessel immediately after or within the initial hours following a dilation procedure. Abrupt closure can result in myocardial infarction if blood flow is not restored in a timely manner. The primary mechanisms of abrupt closures are arterial dissection and/or thrombosis. Restenosis refers to the re-narrowing of an artery after an initial successful angioplasty. Restenosis occurs primarily within the initial six months after angioplasty, and is believed due to the proliferation and migration of the cellular components of the arterial wall.
Endovascular stents are placed in the dilated segment of a vessel lumen to mechanically block the effects of abrupt closure and restenosis. In U.S. Pat. No. 5,514,154, Lau et al. disclose an expandable stent which is relatively flexible along its longitudinal axis. This flexibility facilitates delivery of the stent through tortuous body lumens. Additionally, the stent is stiff and stable enough radially, in an expanded condition, to maintain the patency of a body lumen such as an artery when implanted therein. Such stents have not, however, eliminated abrupt closure and have not eliminated restenosis.
Recent developments have led to stents which can provide anti-thrombogenic and other medications to regions of a blood vessel which have been treated by angioplasty or other interventional techniques. In U.S. Pat. No. 5,464,650, Berg et al. disclose a method for making an intravascular stent by applying to the stent, and in particular to its tissue-contacting surface, a solution which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the solvent. After the solution is applied to the stent, the solvent is then evaporated leaving the polymer/therapeutic agent surface treatment. Berg et al. assert that these devices are capable of providing both short term medication delivery, over the initial hours and days after the treatment, as well as long term medication delivery, over the weeks and months after the treatment.
The process disclosed by Berg et al., which uses a polymeric carrier, is prone to the formation of polymeric surface imperfections during the coating processes. This is especially evident on stents, which generally include many wire like members with interstitial spaces therebetween. The surface imperfections can include strands of drug laden polymeric material hanging loosely from or extending across interstitial spaces between stent portions. The imperfections can also include chunks or thickened coating portions at particular points relative to the rest of the coating. These imperfections, because of their drug delivering capabilities, may cause adverse effects. Loose strands or strands across interstitial spaces may not be secure, and thus, may enter the blood stream and fail to provide local treatment. If these agents are released to locations other than the targeted tissue, unwanted side effects may result. An uneven coating or one with chunks may result in non-uniform treatment of the vessel wall.